Embodiments of the invention relate generally to an apparatus and method of eliminating localized fluctuation artifacts caused by fat signal contamination in MR images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In general it is desirable to image water in an object and suppress fat to avoid fat signal contamination in final images, as known in the art. According to one known method, a spectral spatial water excitation pulse may be applied that generally does not result in artifacts caused by fat. However, such a method is limited in slice thickness and cannot image below approximately 3.1 mm slice thickness.
According to another known technique, fat saturation is in general implemented in a functional MRI (fMRI) pulse sequence to avoid or reduce fat signal contamination in final images. A conventional fat saturation scheme is to use a 90° flip angle, spatially non-selective, 1D RF pulse at fat resonance frequency to rotate fat spins in the whole 3D space (or as far as transmit coil sensitivity goes) to transverse plane. A crusher gradient is then applied to spatially disperse the phase of the rotated fat spins, making their net transverse magnetization in a given imaging voxel negligible. This way of pretreating fat spins before playing out the subsequent water excitation pulse functions as if the fat signals were “saturated” and would not appear in the water signal based final image, thus the name fat saturation.
This conventional fat saturation scheme relies on a sufficiently strong crusher gradient field to remove fat signals generated by the spatially non-selective RF pulse. At or close to the isocenter of the MR magnet, all X, Y, and Z gradient fields have good linearity in their respective axes and generating a strong crusher gradient in this area is therefore a relatively easy task. However, for locations further away from isocenter in the superior-inferior direction, gradient linearity becomes increasingly worse and in some locations (e.g., >20 cm away from isocenter), gradients at some or all axes can become negligibly small, failing to crush out the fat signals in these areas. For human brain imaging, areas with negligible crusher gradients are below the subject's head, which can include neck and chest areas. Fat spins (or water spins under a B0 frequency offset that equals the fat-water frequency difference) in these areas excited by the spatially non-selective fat saturation pulse (assuming axial scan planes), although not re-flipped by the subsequent water excitation pulse, produce artifactual signals that are aliased to contaminate the in-plane water signal.
In the image domain, the artifactual signals affect one or multiple localized voxels because the imaging gradients that encode these out-of-plane signals have zero or weak amplitude as well. For dynamic imaging that involves acquisition of series of images, such as fMRI, and an artifact manifests itself as an additional temporal fluctuation in one or multiple voxels due to motion and/or system instability in the out-of-plane areas. The locations of voxels with the additional fluctuation are coherent through slices—therefore one or multiple dark bands in the slice direction can appear in the sagittally or coronally reformatted signal-to-temporal-fluctuation-noise (SFNR) map. In fMRI applications, the additional temporal fluctuation can result in false activation points in the brain activation map, which can also largely affect the accuracy of any subsequent data analysis.
It would therefore be desirable to have a system and method capable of reducing localized signal fluctuation.